Additive fabrication processes for producing three dimensional objects are well known. Additive fabrication processes utilize computer-aided design (CAD) data of an object to build three-dimensional parts. These three-dimensional parts may be formed from liquid resins, powders, or other materials.
A non-limiting example of an additive fabrication process is stereolithography (SL). Stereolithography is a well-known process for rapidly producing models, prototypes, patterns, and production parts in certain applications. SL uses CAD data of an object wherein the data is transformed into thin cross-sections of a three-dimensional object. The data is loaded into a computer which controls a laser that traces a pattern of a cross section through a liquid radiation curable resin composition contained in a vat, solidifying a thin layer of the resin corresponding to the cross section. The solidified layer is recoated with resin and the laser traces another cross section to harden another layer of resin on top of the previous layer. The process is repeated layer by layer until the three-dimensional object is completed. When initially formed, the three-dimensional object is, in general, not fully cured, and is called a “green model.” Although not required, the green model may be subjected to post-curing to enhance the mechanical properties of the finished part. An example of an SL process is described in U.S. Pat. No. 4,575,330, which is hereby incorporated by reference.
There are several types of lasers used in stereo lithography, traditionally ranging from 193 nm to 355 nm in wavelength, although other wavelength variants exist. The use of gas lasers to cure liquid radiation curable resin compositions is well known. The delivery of laser energy in a stereolithography system can be Continuous Wave (CW) or Q-switched pulses. CW lasers provide continuous laser energy and can be used in a high speed scanning process. However, their output power is limited which reduces the amount of curing that occurs during object creation. As a result the finished object will need additional post process curing. In addition, excess heat could be generated at the point of irradiation which may be detrimental to the resin. Further, the use of a laser requires scanning point by point on the resin which can be time-consuming.
Other methods of additive fabrication utilize lamps or light emitting diodes (LEDs). LEDs are semiconductor devices which utilize the phenomenon of electroluminescence to generate light. At present, LED UV light sources currently emit light at wavelengths between 300 and 475 nm, with 365 nm, 390 nm, 395 nm, 405 nm, and 415 nm being common peak spectral outputs. See textbook, “Light-Emitting Diodes” by E. Fred Schubert, 2nd Edition, © E. Fred Schubert 2006, published by Cambridge University Press, for a more in-depth discussion of LED UV light sources.
Many additive fabrication applications require a freshly-cured part, aka the “green model” to possess high mechanical strength (modulus of elasticity, fracture strength). This property, often referred to as “green strength,” constitutes an important property of the green model and is determined essentially by the nature of the liquid radiation curable resin composition employed in combination with the type of apparatus used and degree of exposure provided during part fabrication. Other important properties of a stereolithographic resin composition include a high sensitivity for the radiation employed in the course of curing and a minimum amount of curl or shrinkage deformation, permitting high shape definition of the green model. Of course, not only the green model but also the final cured article should have sufficiently optimized mechanical properties.
For select additive fabrication applications in the aerospace or automotive industries, for example, three-dimensional solid parts are subjected to the high force loads of a wind tunnel, or the extreme temperatures of a location proximate to heat-generating componentry. In such applications, designers and engineers require a three-dimensional solid part created via additive fabrication to maintain its structural integrity and minimize deflection. Thus three-dimensional parts made from photopolymerizable compositions must possess ceramic-like material properties, such as high strength, stiffness, and heat resistance.
“Filled” liquid radiation curable resins have long been used in the field in an attempt to meet these specialized application design criteria. That is, high amounts of inorganic filler, such as silica (SiO2) have been imparted into traditional “unfilled” liquid radiation curable resins due to the filler's positive impact on the strength and stiffness of the three-dimensional object produced therefrom. Such filled liquid radiation curable compositions are known in the art of additive fabrication, and are disclosed in, e.g., U.S. Pat. No. 5,972,563 (Issued Oct. 26, 1999), U.S. Pat. No. 5,989,475 (Issued Nov. 23, 1999), U.S. Pat. No. 6,287,745 (Issued Sep. 11, 2001), U.S. Pat. No. 6,742,456 (Issued Jun. 1, 2004), U.S. Pat. Pub. No. 20020045126, U.S. Pat. Pub. No. 20040077745, and U.S. Pat. Pub. No. 20050101684, all of which are hereby incorporated by reference. While the aforementioned patents disclose fundamental filled liquid radiation curable compositions, none discuss or teach compositions sufficiently and simultaneously overcoming the several drawbacks typically associated with their use.
Thus, highly filled compositions present several challenges to the formulator of liquid radiation curable resins for additive fabrication. Heretofore no filled liquid radiation curable composition for additive fabrication existed that could both yield three-dimensional parts possessing excellent mechanical properties, yet simultaneously avoid: (1) a high initial viscosity, (2) a poor viscosity stability, and (3) a tendency to phase separate, resulting in phenomena known as either “soft pack” or “hard pack.”
The first long-felt problem with filled liquid radiation curable resin compositions for additive fabrication is that as the amount of filler increases, the viscosity of the resin also usually increases, resulting in decreased workability and processing speed. Highly viscous resins particularly retard the processing speed in vat-based additive fabrication systems such as stereolithography. Existing resins are sufficiently flow-resistant such that they will not readily form a smooth layer of liquid photo curable resin over the just formed solid layer to ensure accurate cure by actinic radiation. Consequently, a recoating operation has traditionally been used to simultaneously place and mechanically smooth a fresh layer of resin over a previously cured layer prior to exposure with actinic radiation. In one non-limiting example, this recoating operation has traditionally been performed by means of a “recoating blade.” A recoating blade design is discussed in, for example, Chapman et al., U.S. Pat. No. 5,626,919, assigned to DSM IP Assets, B.V.
Even with a recoating operation, however, low viscosity remains an important characteristic of the resin. The filled liquid radiation curable resin composition's viscosity affects the time it takes to equilibrate as a smooth, even surface after the recoating step. Consequently, a programmed “dwell time” has been traditionally used between the end of the recoating operation and the beginning of the exposure of the next layer of resin to appropriate imaging radiation. Both the recoating operation and the dwell time dramatically increase the process time of a typical vat-based additive fabrication process.
Additionally, the viscosity of the liquid radiation curable resin also affects the time and difficulty associated with preparing a recently-cured part for post processing operations. In a vat-based additive fabrication process, upon build completion of a three-dimensional solid part, the solidified portions are removed from the liquid uncured resin. A highly viscous resin will be more difficult to separate from the cured part, wherein a resin of substantially low viscosity will be removed without significant effort. Thus, low viscosity resins reduce the time required to clean a part in order to prepare it for post processing operations.
Second, while the importance of filled liquid radiation curable compositions for additive fabrication with a sufficiently low initial viscosity is significant, it is equally as important to conduct additive fabrication processes with a resin having sufficient viscosity stability over time. Filled liquid radiation curable resins for additive fabrication possess a well-known amplified tendency to increase in viscosity over time than versus traditional unfilled resins. This exacerbates the aforementioned problems associated with the high initial viscosities of filled compositions, resulting in increasingly less efficient, more costly additive fabrication processes over time.
Additionally, highly filled compositions are usually not as thermal- or photo-stable as non-filled liquid radiation curable resins for additive fabrication. Photo stability is the ability of a liquid radiation curable resin to maintain its viscosity after exposure to ambient light and undesirable light scattering in additive fabrication machines. Thermal stability is the ability of a liquid radiation curable resin to maintain its viscosity after exposure to elevated temperatures, which are known to accelerate cationic polymerization. Because liquid radiation curable resins for additive fabrication include reactive species that are responsive to undesirable ambient light scattering that occurs as a result of contact with crystallized filled particles, partial uncontrolled polymerization occurs in the liquid radiation curable resin after it is exposed to light. This small amount of uncontrolled polymerization, over time, is accelerated if the resin is stored at elevated temperatures. These factors cause the viscosity of the liquid radiation curable resin to increase gradually—but significantly—over time. Thus achieving sufficient viscosity stability is particularly challenging in highly filled liquid radiation curable resins because of additional light scattering effects caused by the filler.
A third problem traditionally associated with filled liquid radiation curable compositions for additive fabrication is their tendency to phase separate in storage or a vat over time. This phase separation, whereby the inorganic filler loses its state of homogeneous suspension in the surrounding liquid radiation curable resin, results in a vat made up of a bifurcated composition: (1) a low-viscosity, largely unfilled top portion, and (2) a supersaturated, high-viscosity bottom portion. The resin in the top portion of a vat is not able to produce cured parts possessing sufficient strength and stiffness (because of a lack of filled component present in the composition), while the bottom portion is devoid of any suitability for additive fabrication at all (due to an excess of filled particles). Despite the historical inclusion of inorganic particles designed for anti-sedimentation, existing filled liquid radiation curable compositions for additive fabrication invariably eventually settle, either into a “soft pack” or a “hard pack.” In the more benign settling phenomenon, a soft pack, the settled filler forms a waxy portion at the bottom of a storage container or vat. The settled filler is frequently surrounded by partially polymerized resin, resulting in the wax-like consistency. Although re-assimilation into the liquid radiation curable resin as a whole is possible, it requires frequent and often vigorous recirculation. This is a time- and energy-consuming maintenance process, and still does not obviate a resin's eventual viscosity increase, due to the rampant partial polymerization.
Still other filled liquid radiation curable compositions for additive fabrication settle into an undesirable “hard pack.” Hard pack occurs whereby the inorganic filler settles to the bottom of a storage container or vat, forming a concrete-like piece or pieces. These pieces must be broken up by a drill or similar apparatus, and are typically unable to be re-assimilated into the liquid radiation curable resin as a whole. This shortens the shelf-life of such resins, or results in changing and inconsistent properties in the cured parts made therefrom, due to the changing amounts of filler component which are not re-miscible into the solution. Thus, it is especially desirable to formulate a liquid radiation curable resin composition for additive fabrication which, in addition to the other required performance characteristics mentioned above, possesses superior anti-sedimentation capabilities.
Various other patents or patent publications also describe using inorganic filled compositions comprising, inter alia, silica microparticles and/or nanoparticles, among them:
U.S. Pat. No. 6,013,714 (Haruta et al.), assigned to DSM IP Assets, B.V., which describes additive fabrication processes that utilizes filled resins to performing a combination of steps, including (1) applying a thin layer of resin on a supporting stage; (2) selectively irradiating the thin layer of resin as to cure a selected part of said resin; (3) applying a further thin layer of resin; and repeating steps (2) and (3) as to obtain a three dimensional shape of a plurality of cured layers, optionally combined with either one of the steps of washing and post-curing the three dimensional shape, as to obtain the mold, wherein the resin composition is formulated from constituents comprises at least one photoreaction monomer at least one photoinitiator at least one filler.
U.S. Pat. Pub. No. 20050040562 (Steinmann et al.), assigned to 3D Systems, Inc., which describes a process for forming a three-dimensional article by stereolithography, said process comprising the steps: 1) coating a thin layer of a liquid radiation-curable composition onto a surface of said composition including at least one filler comprising silica-type nano-particles suspended in the radiation-curable composition: 2) exposing said thin layer imagewise to actinic radiation to form an imaged cross-section, wherein the radiation is of sufficient intensity to cause substantial curing of the thin layer in the exposed areas; 3) coating a thin layer of the composition onto the previously exposed imaged cross-section; 4) exposing said thin layer from step (3) imagewise to actinic radiation to form an additional imaged cross-section, wherein the radiation is of sufficient intensity to cause substantial curing of the thin layer in the exposed areas and to cause adhesion to the previously exposed imaged cross-section; 5) repeating steps (3) and (4) a sufficient number of times in order to build up the three-dimensional article.
U.S. Pat. Pub. No. 20120251841 (Southwell et al.), assigned to DSM IP Assets B.V., which describes liquid radiation curable resins for additive fabrication comprising an R-substituted aromatic thioether triaryl sulfonium tetrakis(pentafluorophenyl)borate cationic photoinitiator and silica nanoparticles. Also disclosed is a process for using the liquid radiation curable resins for additive fabrication and three-dimensional articles made from the liquid radiation curable resins for additive fabrication.
From the foregoing, it is evident that no filled liquid radiation curable compositions for additive fabrication exist that are suitable for producing cured components having adequate application-specific heat resistance and structural rigidity, while simultaneously overcoming the long-felt but unsolved industry needs of providing the requisite low initial viscosity, high viscosity stability, and high phase-separation resistance.